Prevent that removeActivityResultListener() is causing an ConcurrentModificationException (#19702)

shell/platform/android/io/flutter/embedding/engine/FlutterEnginePluginRegistry.java

@@ -686,7 +686,7 @@ class FlutterEnginePluginRegistry
     boolean onActivityResult(int requestCode, int resultCode, @Nullable Intent data) {
       boolean didConsumeResult = false;
       for (io.flutter.plugin.common.PluginRegistry.ActivityResultListener listener :
-          onActivityResultListeners) {
+          new HashSet<>(onActivityResultListeners)) {
         didConsumeResult =
             listener.onActivityResult(requestCode, resultCode, data) || didConsumeResult;
       }

shell/platform/android/test/io/flutter/embedding/engine/FlutterEnginePluginRegistryTest.java

@@ -6,11 +6,18 @@ import static org.junit.Assert.assertFalse;
 import static org.mockito.Mockito.mock;
 import static org.mockito.Mockito.when;
 
+import android.app.Activity;
 import android.content.Context;
+import android.content.Intent;
 import androidx.annotation.NonNull;
+import androidx.lifecycle.Lifecycle;
 import io.flutter.embedding.engine.loader.FlutterLoader;
 import io.flutter.embedding.engine.plugins.FlutterPlugin;
+import io.flutter.embedding.engine.plugins.activity.ActivityAware;
+import io.flutter.embedding.engine.plugins.activity.ActivityPluginBinding;
+import io.flutter.plugin.common.PluginRegistry;
 import io.flutter.plugin.platform.PlatformViewsController;
+import java.util.concurrent.atomic.AtomicBoolean;
 import org.junit.Test;
 import org.junit.runner.RunWith;
 import org.robolectric.RobolectricTestRunner;
@@ -58,6 +65,49 @@ public class FlutterEnginePluginRegistryTest {
     assertEquals(0, fakePlugin2.detachmentCallCount);
   }
 
+  @Test
+  public void activityResultListenerCanBeRemovedFromListener() {
+    Context context = mock(Context.class);
+
+    FlutterEngine flutterEngine = mock(FlutterEngine.class);
+    PlatformViewsController platformViewsController = mock(PlatformViewsController.class);
+    when(flutterEngine.getPlatformViewsController()).thenReturn(platformViewsController);
+
+    FlutterLoader flutterLoader = mock(FlutterLoader.class);
+    Activity activity = mock(Activity.class);
+    Lifecycle lifecycle = mock(Lifecycle.class);
+    Intent intent = mock(Intent.class);
+    AtomicBoolean isFirstCall = new AtomicBoolean(true);
+
+    // setup the environment to get the required internal data
+    FlutterEnginePluginRegistry registry =
+        new FlutterEnginePluginRegistry(context, flutterEngine, flutterLoader);
+    FakeActivityAwareFlutterPlugin fakePlugin = new FakeActivityAwareFlutterPlugin();
+    registry.add(fakePlugin);
+    registry.attachToActivity(activity, lifecycle);
+
+    // The binding is now available via `fakePlugin.binding`: Create and add the listeners
+    FakeActivityResultListener listener1 =
+        new FakeActivityResultListener(isFirstCall, fakePlugin.binding);
+    FakeActivityResultListener listener2 =
+        new FakeActivityResultListener(isFirstCall, fakePlugin.binding);
+
+    fakePlugin.binding.addActivityResultListener(listener1);
+    fakePlugin.binding.addActivityResultListener(listener2);
+
+    // fire the onActivityResult which should invoke both listeners
+    registry.onActivityResult(0, 0, intent);
+
+    assertEquals(1, listener1.callCount);
+    assertEquals(1, listener2.callCount);
+
+    // fire it again to check if the first called listener was removed
+    registry.onActivityResult(0, 0, intent);
+
+    // The order of the listeners in the HashSet is random: So just check the sum of calls
+    assertEquals(3, listener1.callCount + listener2.callCount);
+  }
+
   private static class FakeFlutterPlugin implements FlutterPlugin {
     public int attachmentCallCount = 0;
     public int detachmentCallCount = 0;
@@ -72,4 +122,50 @@ public class FlutterEnginePluginRegistryTest {
       detachmentCallCount += 1;
     }
   }
+
+  private static class FakeActivityAwareFlutterPlugin implements FlutterPlugin, ActivityAware {
+    public ActivityPluginBinding binding;
+
+    @Override
+    public void onAttachedToEngine(@NonNull FlutterPluginBinding binding) {}
+
+    @Override
+    public void onDetachedFromEngine(@NonNull FlutterPluginBinding binding) {}
+
+    @Override
+    public void onAttachedToActivity(final ActivityPluginBinding binding) {
+      this.binding = binding;
+    }
+
+    @Override
+    public void onDetachedFromActivityForConfigChanges() {}
+
+    @Override
+    public void onReattachedToActivityForConfigChanges(final ActivityPluginBinding binding) {}
+
+    @Override
+    public void onDetachedFromActivity() {}
+  }
+
+  private static class FakeActivityResultListener implements PluginRegistry.ActivityResultListener {
+    public int callCount = 0;
+    private final AtomicBoolean isFirstCall;
+    private final ActivityPluginBinding binding;
+
+    public FakeActivityResultListener(AtomicBoolean isFirstCall, ActivityPluginBinding binding) {
+      this.isFirstCall = isFirstCall;
+      this.binding = binding;
+    }
+
+    @Override
+    public boolean onActivityResult(
+        final int requestCode, final int resultCode, final Intent data) {
+      callCount++;
+      if (isFirstCall.get()) {
+        isFirstCall.set(false);
+        binding.removeActivityResultListener(this);
+      }
+      return false;
+    }
+  }
 }